Remote Procedure Calls CS 707

Transcription

1 Remote Procedure Calls CS 707

2 Motivation Send and Recv calls I/O Goal: make distributed nature of system transparent to the programmer RPC provides procedural interface to distributed services CS 707 2

3 Figure 4.5 Request-reply communication. Client Server DoOperation (wait) (continuation) Request message Reply message GetRequest execute request SendReply Instructor s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 2 (2nd impression) Addison-Wesley Publishers

4 Figure 5.2 Stub procedures. Client computer Client process Server computer Server process Local call Local return Marshal arguments Unmarshal results Send Request Receive Reply Receive Request Send Reply Select procedure Unmarshal arguments Marshal results Execute procedure Return Client Client stub procedure Communication module Communication module Service Despatcher procedure Server stub Instructor s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 2 (2nd impression) Addison-Wesley Publishers

5 Issues in RPC Parameter Passing marshalling big endian vs little endian J I L L L L I J Intel 386 Sparc CS 707 3

6 Parameter Passing Passing arrays in C by reference Canonical forms SUN XDR, Xerox Courier Passing pointers? CS 707 4

7 Figure 4.1 XDR message. 4 bytes 5 length of sequence "Smit" "h " Smith 6 length of sequence "Lond" "on " London The message is: Smith, London, CARDINAL 4 Instructor s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 2 (2nd impression) Addison-Wesley Publishers

8 Figure 4.6 Request-reply message structure. messagetype requestid procedureid arguments (Request, Reply) CARDINAL CARDINAL (* flattened list*) Instructor s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 2 (2nd impression) Addison-Wesley Publishers

9 Figure 4.7 RPC protocols. Name Messages sent by Client Server Client R Request RR Request Reply RRA Request Reply Acknowledge reply Instructor s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 2 (2nd impression) Addison-Wesley Publishers

10 Handling Failures Types of failure client unable to locate server request message lost reply message lost server crashes after receiving a request client crashes after sending a request CS 707 5

11 Handling failures Client cannot locate server Reasons server crashed client compiled using old version of server interface RPC system must be able to report error to client loss of transparency CS 707 6

12 Handling failures Lost request message retransmit a fixed number of times Lost reply message client retransmits request server choices filter duplicates hold on to results until ACK re-execute procedure service should be idempotent so that it can be safely repeated CS 707 7

13 Handling failures REQ Server crashes REQ REQ REP Recv exec reply NO REP Recv exec crash NO REP Recv crash Client cannot tell difference CS 707 8

14 Handling failures Server crashes at least once (keep trying till server comes up) at most once (return immediately) exactly once impossible to achieve ONC RPC (Sun) uses at least once semantics if a RPC is successful and zero or more semantics if call fails CS 707 9

15 Retry request message Delivery guarantees Duplicate filtering Re-execute procedure or retransmit reply RPC call semantics No Not applicable Not applicable Maybe Yes No Re-execute procedure At-least-once Yes Yes Retransmit reply At-most-once Instructor s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 2 (2nd impression) Addison-Wesley Publishers

16 Client crashes Handling failures if a client crashes before RPC returns, we have an orphan computation at server wastes resources, could also start other remote computations orphan detection reincarnation (client broadcasts new epoch when it comes up) expiration (RPC has fixed amount of time T to do work) CS

17 Binding Dynamic Servers register service with binder withdraw Client lookup address of service SUN RPC portmapper runs on every host CS

18 Figure 5.3 Binder Interface. PROCEDURE Register (servicename:string; serverport:port; version:integer) causes the binder to record the service name and server port of a service in its table, together with a version number. PROCEDURE Withdraw (servicename:string; serverport:port; version:integer) causes the binder to remove the service from its table. PROCEDURE LookUp (servicename:string; version:integer): Port the binder looks up the named service and returns its address(or set of addresses) if the version number agrees with the one stored in its table. Instructor s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 2 (2nd impression) Addison-Wesley Publishers

19 RPC protocols Connection-oriented or connection-less connectionless has better performance on LANs UDP/IP or roll your own protocol specialized protocol better but more effort large RPCs have to be broken up into multiple packets CS

20 Interface definition languages SUN XDR rpcgen (IDL compiler) creates client and server stub procedures, header files, despatcher, server main procedure stubs use XDR for marshalling and unmarshalling CS

21 Figure 5.1 Levels in the client software. Client program User package Client stub procedures User package interface RPC interface Communication handler 5 Instructor s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 2 (2nd impression) Addison-Wesley Publishers

22 Figure 5.4 Files interface in Sun XDR. /* FileReadWrite service interface definition in file FileReadWrite.x */ const MAX = 1000; typedef int FileIdentifier; typedef int FilePointer; typedef int Length; struct Data { int length; char buffer[max]; }; struct writeargs { FileIdentifier f; FilePointer position; Data data; }; struct readargs { FileIdentifier f; FilePointer position; Length length; }; program FILEREADWRITE { version VERSION { void WRITE(writeargs)=1; Data READ(readargs)=2; }=2; } = 9999; Instructor s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 2 (2nd impression) Addison-Wesley Publishers

23 Figure 5.5 C program for client in Sun RPC. /* File : C.c - Simple client of the FileReadWrite service. */ #include <stdio.h> #include <rpc/rpc.h> #include "FileReadWrite.h" main(int argc, char ** argv) { CLIENT *clienthandle; char *servername = "coffee"; readargs a; Data *data; clienthandle= clnt_create(servername, FILEREADWRITE, VERSION, "udp"); /* creates socket and a client handle*/ if (clienthandle==null){ clnt_pcreateerror(servername); /* unable to contact server */ exit(1); } a.f = 10; a.position = 100; a.length = 1000; data = read_2(&a, clienthandle);/* call to remote read procedure */ clnt_destroy(clienthandle); /* closes socket */ } Instructor s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 2 (2nd impression) Addison-Wesley Publishers

24 Figure 5.6 C program for server procedures in Sun RPC. /* File S.c - server procedures for the FileReadWrite service */ #include <stdio.h> #include <rpc/rpc.h> #include"filereadwrite.h" void * write_2(writeargs *a) { /* do the writing to the file */ } Data * read_2(readargs * a) { static Data result; /* must be static */ result.buffer =... /* do the reading from the file */ result.length =... /* amount read from the file */ return &result; } Instructor s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 2 (2nd impression) Addison-Wesley Publishers